The arrangement of two chips one above the other and their electrical interconnection is also referred to as “vertical circuit integration”. One possibility for producing the electrical connection between the first and second chips is the use of bonding wires. In this connection method, one of the two chips has a significantly larger base area than the other. Each of the two chips has bonding pads on its active main side, said bonding pads being located in a region at the edge, for example, in the case of the larger chip, so that the smaller chip cannot be placed into the central region that is not provided with bonding pads. Finally, the electrical connection is produced by bonding wires between respective bonding pads.
Another possibility consists in using conductive adhesives or soldering balls for producing the electrical connection. In both variants, the active main areas of the first and second chips face one another, so that respective contact areas are located opposite one another. Point contacts are then produced by means of the conductive adhesive or the solder balls. Shear forces on account of thermal stresses can therefore lead to an impairment of the electrical contact.
In the variants described hitherto, the diameter of the bonding pads or of the contact areas is between 70 and 100 μm. The distance between two bonding pads or external contact areas is likewise of the order of magnitude mentioned.
The provision of additional electrical functions in the case of an electrical connection by means of bonding wires, conductive adhesives or solder balls requires extensive design changes with regard to the arrangement of the first and second chips on one another and the electrical connection of individual bonding pads or external contact areas.
An alternative connection method for producing an electrical connection between the first and second chips is the so-called “diffusion soldering method”. In the latter, the first and second chips are arranged with their active main areas in relation to one another. Situated on a respective active main area is a first and second metalization, which face one another. The first and second metalizations may be embodied in the form of a copper layer with a respective thickness of 1 to 5 μm. In order to produce an electrical connection, an additional thin solder layer, e.g. made of tin, with a thickness of between 0.5 and 3 μm is introduced between the first and second metalizations. The total thickness comprising the first and second metalizations and the solder layer lying in between is typically less than 10 μm. Thus, in comparison with the connection methods mentioned in the introduction, here an additional thin metal plane is produced which can be patterned in a range of 1 μm on account of its small thickness.